The present application includes a computer program listing appendix on compact disc. Two duplicate compact discs are provided herewith. Each compact disc contains an ASCII text file of the computer program listing as follows:
Filename: NSL-Pneumatic Text File for Patent.txt
Size: 223,863 bytes
Date Created: Nov. 6, 2001
The computer program listing appendix is hereby expressly incorporated by reference in the present application
I. Field of the Invention
The present invention relates generally to ophthalmic tonometers for measuring intraocular pressure (IOP) of a patient""s eye, and more particularly to non-contact tonometers of the type having a piston movable in a compression stroke relative to a cylinder to compress air within a compression volume of the non-contact tonometer, and a discharge tube in flow communication with the compression volume for directing an air pulse at the patient""s eye to deform the cornea.
II. Description of the Related Art
Non-contact tonometers are diagnostic instruments widely used by ophthalmologists and medical personnel for measuring the internal fluid pressure within the eye (intraocular pressure or IOP), often to screen patients for elevated IOP associated with glaucoma. Non-contact tonometers typically operate by directing a fluid pulse at the eye and observing deformation of the cornea. In conventional apparatus of the prior art, a fluid pump having a solenoid-driven piston compresses fluid within a compression volume, and a fluid discharge tube in communication with the compression volume and aligned with the patient""s eye delivers a fluid pulse to the eye that deforms the cornea from its normal convex state, through a flattened state known as xe2x80x9capplanation,xe2x80x9d to a concave state. When the fluid pulse dissipates, the cornea returns to its normal convex state. The deformation is monitored by opto-electronic means, and a quantity such as the plenum pressure at the moment of applanation or the time required to achieve applanation is measured and correlated to IOP.
Heretofore, non-contact tonometers have been primarily bulky xe2x80x9ctable topxe2x80x9d instruments that are not easily portable. In practice, the patient sits at one end of the instrument with his or her head steadied by a forehead brace, and the operator sits at the opposite end to align the instrument relative to the eye and administer the test. The instrument, which contains precisely aligned optical components, remains stationary on the table except for a test portion that moves relative to a base of the instrument for alignment purposes. The primary design criterion for pump systems in non-contact tonometers of the prior art has been the capacity of the pump system to deliver the necessary force to the cornea to cause applanation, even where there exists high IOP; size and weight of the pump system have been secondary design considerations.
The desirability of a smaller, lightweight instrument for measuring IOP has been recognized for some time, as evidenced by the development of hand-held xe2x80x9ccontactxe2x80x9d type tonometers. See for example, U.S. Pat. Nos. 4,192,317; 4,622,459; 4,747,296; and 5,174,292. Because a portion of the tonometer physically contacts the cornea, these instruments are generally regarded as being less comfortable to the patient than the noncontact variety described above, and there is an increased risk of infection because viruses and bacteria can be transferred from one patient to the next. Moreover, an operator""s skill in testing can have a significant impact upon measurement results, thus rendering these instruments poorly suited for use by general medical practitioners.
U.S. Pat. No. 4,724,843 describes a portable non-contact tonometer that includes a carrying case 102 for housing a pump used to generate a fluid pulse, and a detachable hand-held unit 100 connected to the pump by a flexible connection line 104 enclosing a fluid conduit. Thus, only a portion of the instrument is hand-held, with the remainder of the instrument being large and heavy. The non-contact tonometer described in U.S. Pat. No. 4,724,843 is evidence that the size and weight of the pump mechanism presents a challenge for those attempting to design a truly hand-held non-contact tonometer that is compact and lightweight, yet is powerful enough, if necessary, to cause applanation of elevated IOP eyes.
The desire for a lighter tonometric pump mechanism runs counter to some of the advantages presented by a large diameter piston. These advantages include a shorter stroke length to achieve a target plenum pressure, and greater opposing force due to increased surface area for quickly stopping the piston after the driving solenoid has been switched off, thereby reducing unnecessary and uncomfortable xe2x80x9cexcess puffxe2x80x9d delivered to the patient""s eye. In fact, it can be shown that of the variables relating to pump mechanism design (i.e. piston diameter, piston mass, plenum volume, orifice size, etc.), the piston diameter has the greatest influence on stroke length, impulse delivered to the eye, and peak plenum pressure.
Therefore, it is an object of the present invention to provide a non-contact tonometer with a piston having a piston diameter that is optimal for achieving a target plenum pressure within a given maximum stroke length without adding unwanted size and mass to the tonometer.
It is another object of the present invention to provide a systematic method for selecting a piston diameter in keeping with the aforementioned object.
In furtherance of these and other objects, a simulation software program is disclosed to numerically simulate the dynamics associated with a tonometer pump compression stroke for a plurality of piston diameters, whereby an optimal piston diameter or range of piston diameters is selected in view of stoke length limitations and target applanation pressure requirements. Consequently, the present invention encompasses both an improvement to a non-contact tonometer, and a method for selecting the piston diameter for a tonometer pump system using a numerical simulation technique.